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Homework Help: Kinetic energy/rotational kinetic energy

  1. May 28, 2009 #1
    1. The problem statement, all variables and given/known data

    Imagine two scenarios. The system in both is the object and ruler.

    In case 1, a rectangular object undergoes a collision with a ruler, which is initially at rest. It strikes the ruler on its center of mass.

    In case 2, it is the same situation except the object strikes the ruler in some point between its center of mass and the end of the ruler.

    In which case does the ruler have greater kinetic energy and why? Explain in terms of conserved quantities.

    2. Relevant equations

    3. The attempt at a solution

    The net torque and net force in both cases would be zero by the laws of conservation of linear and angular momentum.

    I'm reasoning that the kinetic energy in case 2 would be greater because in case 2, both rotational kinetic energy and kinetic energy are conserved while in case 1, only kinetic energy is conserved.
    Last edited: May 28, 2009
  2. jcsd
  3. May 28, 2009 #2
    Sorry about the bump, but I have an exam soon and an answer to question would really help me piece things together.
  4. May 29, 2009 #3
    im going to give you my thoughts because you require some kind of answer now

    this may not be correct, but it may help...so take it with a grain of salt...


    there is a net force due to forward motion of ruler

    there is a 0 net torque as the object strikes the ruler at its CG so there is no moment

    and kenitic energy is present due to the forward motion of the ruler


    there is a net force due to forward motion of ruler

    there is a net force as the object strikes ruler a distance away from the CG of the ruler, so a moment is created allowing the ruler to spin

    there is a higher kenetic energy in this case as the ruler is moving forward and spinning

    hope that helps
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